Method for real-time communication between a number of network subscribers in a communication system using ethernet physics, and a corresponding communication system using ethernet physics

ABSTRACT

For real-time communication between a number of network subscribers in a communication system using Ethernet physics, the slave units are synchronized to the master unit in that each slave unit is clocked via a respective timer with a predetermined overall cycle time, which timer is set cyclically by the reception of respective slave-specific synchronization information which is determined by the master unit. In this case, each timer in a slave unit automatically starts a new cycle once the predetermined overall cycle time has elapsed, even in the absence of the respective synchronization information. Access control for the transmission mode and reception mode between the network subscribers is provided using a timeslot access method.

BACKGROUND OF THE INVENTION

[0001] As widely differing technical systems are increasingly networked,there is a growing requirement for standardized structures in industry.In this context, it is also desirable to be able to couple any desiredappliances locally. In order to provide open systems for networking, itis necessary to provide simple and cost-effective communicationmechanisms which enable industrial appliances to have a networkingcapability. In particular, this requirement also exists in the contextof the coupling of drive components, such as drive controllers, powersections and transmitters, for numerically controlled machine tools androbots, in which a number of interpolating axes must be operatedsynchronously.

[0002] In present-day high-performance drive systems, the interfaces totransmitters and power sections are in the form of analog signalinterfaces. This, however, involves considerable restrictions relatingto the spatial distribution capability, since the susceptibility tointerference from EMC effects (EMC stands for electromagneticcompatibility) increases with the cable length. If the performancerequirements are low, proprietary company serial digital transmissionsystems are generally used. Where the performance requirements are high,the communication between the drive controller and the movementcontroller is provided by proprietary-company serial data transmissionsystems.

[0003] Recently, with regard to the requirement to provide industrialappliances with a networking capability the ETHERNET (data transmissionrate 10 Mbps), in particular the FAST ETHERNET (data transmission rate100 Mbps—IEEE Standard 802.3-1998) data transmission technology which isknown from office technology, has been becoming increasingly important.This is due to the fact that this development represents an undefinedStandard with regard to compatibility and, furthermore, is available atlow cost, since appropriate interface hardware is being produced inlarge quantities, owing to the widespread use in the field of personalcomputers. Furthermore, ETHERNET networks are already widely used inmany organizations, so that a widely extended infrastructure can alreadybe made use of. These arguments all favor the use of the ETHERNET in thefield of automation technology as well. ETHERNET is generally used inthe field of Local Area Networks LAN, with the most widely usedtransmission protocol being TCP/IP (Transmission ControlProtocol/Internet Protocol).

[0004] Accordingly, the IEEE Standard 802.3, CSMA/CD (Carrier SenseMultiple Access/Collision Detect) is generally used as the accessmethod. In this method, all the network subscribers have equal priority,and any network subscriber is allowed to send a message on the network,generally a bus system at any time. However, a problem occurs in thiscase when two or more subscribers are sending a message at the sametime. In this situation, a collision is identified, and each subscriberinvolved is then assigned a waiting time, which is defined randomly,before another attempt to send the message is made. The term statisticalaccess method is therefore used.

[0005] The requirements for the performance of communication systems forautomation technology are particularly stringent, for example whencoupling drive components. When interchanging data between transmitters,power sections and a drive controller, the data transmission time, whichis included in the control loop as a dead time, is a particularlyimportant parameter. The shorter this dead time, the better the dynamicresponse which can be achieved by the control system.

[0006] The connection between movement controllers and drive controllersis also sensitive to dead times, since a control loop is also closed viathis connection. There is thus a problem in particular in the datatransmission time for serial communication systems, which can be solvedonly by an appropriately fast system with a real-time capability, thatis to say a deterministic system. However, the fact that communicationusing ETHERNET networks does not ensure a determined time response runscounter to their use for automation technology. The ETHERNET Standardtherefore does not offer the technical preconditions for real-timecommunications.

[0007] IEEE Standard 802.3 defines a message frame which is not suitablefor this purpose. Since, however, the components for physical ETHERNETsignal transmission are independent of the Standard protocol form, thedeveloper has freedom to choose the protocol form in which the data areto be transmitted. Only Layer 1 (the physical layer) is adopted fromIEEE Standard 802.3.

SUMMARY OF THE INVENTION

[0008] The object of the present invention is to provide a transportprotocol which allows real-time communication while including the widelyused Ethernet physics. According to the present invention, this objectis achieved by a method for real-time communication between a number ofnetwork subscribers in a communication system using Ethernet physics,wherein:

[0009] a master unit and one or more slave units communicate with oneanother by means of messages which are transmitted via the network;

[0010] the messages are interchanged cyclically with equidistantsampling times, in that each slave unit is synchronized to the masterunit by means of a common timebase; and

[0011] access control for the transmission mode and reception mode iscarried out between the network subscribers using a timeslot accessmethod.

[0012] Since the aforementioned applications require both high-precisioncompliance with the real-time condition and a high level of security andreliability in the transmission, the standardized transmission layer 2(message frame and access method) of the (Fast) Ethernet, which does notsatisfy these requirements, is completely redefined by a new messageframe and a new access control method, and the Ethernet physics are thusused as the basis for real-time communication between, for example,drive components. Both the communication between the control unit andthe transmitters and power sections, and the connection to a movementcontroller, can be provided in this way.

[0013] With regard to synchronization between the master and slaveunits, it has been found to be advantageous for the slave units to besynchronized to the master unit by each slave unit being clocked via arespective timer with a predetermined overall cycle time, which is setcyclically by the reception of a respective slave-specificsynchronization information item, which is determined by the masterunit. A master-slave communication architecture is thus used. In orderto allow cyclic data interchange with identical sampling times to beprovided, a common time base is produced for the master and all theslaves. The slaves are synchronized to the master by means ofspecifically defined messages, defined in time, from the master to theslaves, and individually configured timers in the slaves. User datamessages and specific synchronization messages, which contain therespective synchronization information, can be transmitted in this way.Alternatively, the synchronization information can also be integrated ina specifically defined user data message. The stability of thecommunication system can be further enhanced if each timer in a slaveunit automatically starts a new cycle once the predetermined overallcycle time has elapsed, even in the absence of the respectivesynchronization information.

[0014] A timeslot access method, which is initialized by the master inthe network and allows optimum dead-time data transmission, is used forcyclic data transmission for the transmission and reception modes.Accordingly, messages can be monitored precisely for premature ordelayed transmission, or for transmission subject to interference. Forthis purpose, only the master unit, has transmission authorization onthe network for initialization, and reports to each slave unit which hasonly response authorization, via an appropriate slave-specific message.In addition to the overall cycle time, the time slots within the overallcycle time determine when the respective slave unit will receivemessages from the master unit and the times at which it should send itsmessages. It has been found to be advantageous if each slave unit istold the respective synchronization time in the initialization phase.

[0015] Simultaneous and equidistant sampling for a control system can beachieved by storing instantaneous values in each slave unit at a commontime, in particular at the start of a cycle.

[0016] In a preferred embodiment of the method according to the presentinvention, monitoring information is provided in each message which istransmitted by the master unit to a slave unit, by means of whichsecurity or safety functions which are provided in the slave unit can beactivated directly via a second initiation channel.

[0017] The user data can be transported in a message frame which, inaddition to slave addressing and message length information, providesprotection of the data integrity by means, for example, of a CRCchecksum and further security and safety related data areas. The data inthe message frame can be evaluated not only by an application processor,but also by a communication module, which allows a second initiationchannel. For this purpose, each slave unit sends a life signal with eachmessage to the master unit. The absence of this signal indicates thatthe slave system has crashed, and the master unit stops this slave unitin a controlled manner by means of the second initiation channel(without the assistance of the slave unit). Furthermore, in the userdata area of its message to the slave unit, the master unit can initiatea function in the slave unit, and can initiate this simultaneously bymeans of a signal in the second initiation channel. Two-channelinitiation is thus achieved, which is an essential requirement forcertain security or safety applications. The master unit can also send amaster life signal in each of its messages to the slave units. In theabsence of this signal, all the slaves react by stopping their ownfunctions in a controlled manner.

[0018] Although the transmission technology based on the EthernetStandard allows only point-to-point connections, it is also possible toform networks by using network nodes (which are referred to as HUBs) in(fast) Ethernet networks by a number of network subscribers, or eachnetwork subscriber, having one circuit part in order to form networknodes. The circuit part is used for passing on the messages in thedirection of another master unit or further slave units, whereincommunication between network subscribers via network nodes likewisetakes place as claimed in one of the procedures described above.

[0019] Furthermore, separate transmitting and receiving lines betweentwo network subscribers can be used simultaneously. This is done, forexample, based on the rule that all the slave units transmit only in thedirection of the master unit, and receive only messages directed fromthe master unit. This means that a message from the master in thedirection of a slave and a message from a slave in the direction of amaster can be transmitted simultaneously. This allows for full-duplexoperation.

[0020] Real-time communication can be achieved on the basis of acommunication system using Ethernet physics by means of the proceduredescribed above. In this case, hierarchical networks can also beproduced by means of point-to-point connections, connected via networknodes, using Ethernet physics for carrying out real-time communicationin relatively large network topologies. This is also suitable fornetworking or coupling a distributed drive system, in that a firstcommunication system comprises a numerical movement controller as themaster unit and at least one control unit as the slave unit. Eachcontrol unit is used as the master unit for a further communicationsystem, which has at least one power section for driving a motor and anassociated transmission system as slave units.

[0021] Security and safety applications can also be provided since thecommunication between the drive components, such as a control unit,transmitter, power sections and movement controllers, is accomplished bymeans of an existing high-performance transmission system from theoffice communication field, by means of a completely new protocol,master-slave synchronization and a timeslot access method, for areal-time capability.

[0022] The use of the (fast) Ethernet transmission technology includingthe method according to the present invention thus results, inter alia,in the following advantages:

[0023] low-cost line drivers, since large quantities of these are usedin office communication technology—a proven technology which is alsoused in industry;

[0024] the line drivers allow any desired protocol and full-duplexoperation;

[0025] better cable material can be used than for multi-core analoguesignal cables;

[0026] the distances between the components can be greater than whenusing analog signal cables;

[0027] shorter dead times can be achieved by means of high transmissionperformance levels of up to 100 Mbps with fast Ethernet; and

[0028] even complex networks can thus also be constructed.

DRAWINGS

[0029] Further advantages and details of the present invention aredescribed below in conjunction with the figures, in which:

[0030]FIG. 1 shows an outline illustration of a movement controller anda drive appliance having two axes, based on the communication system ofthe invention;

[0031]FIG. 2 shows a timing diagram of the timer synchronization in aslave;

[0032]FIG. 3 shows an illustration of the timeslot access method;

[0033]FIG. 4 shows an illustration of one possible message frame;

[0034]FIG. 5 shows an illustration of one possible function code withina message frame; and

[0035]FIG. 6 shows an outline illustration for a second initiationchannel for security and safety applications.

DETAILED DESCRIPTION OF THE INVENTION

[0036]FIG. 1 shows an example of a movement controller NC and of a driveappliance with two axes, which are coupled by means of a firstcommunication system KOMSYS1 based on Ethernet physics. The drivecontroller R is connected to its power sections A1, A2 and transmittersS1, S2 by means of its own independent communication system KOMSYS2.Each power section A1 and A2 is used to drive the respective motor M1,M2, to which respective sensors S1, S2 in the form of transmittersystems are in turn connected, in order to detect the current, rotationspeed and position and orientation information required for position andorientation control.

[0037] With regard to the first communication system KOMSYS1, themovement controller NC represents the master unit, which communicateswith a number of controllers R (FIG. 1 shows only one control group, forthe sake of clarity) as slave units. In the second communication systemKOMSYS2, the controller R forms the master unit, while the actuators A1,A2 and sensors S1, S2 represent the slave units.

[0038] The network subscribers are connected via fast Ethernet linedrivers PHY within each network subscriber. By way of example, the linedriver modules PHY of the fast Ethernet are used for physical datatransmission, on the basis of a copper cable having at least four cores,or a two-conductor optical waveguide.

[0039] In order to network different communication systems, all thenetwork subscribers, or some of them (in the present case, the movementcontroller NC and the actuators A1, A2) have circuit parts HUB which areconnected downstream from the line driver modules PHY and are used topass on the messages in the direction of the master or further slaves.It is thus possible to form a hierarchical network, as shown in FIG. 1.

[0040] The messages are passed via the line drivers PHY and any networknodes HUB to the respective protocol modules Kom, which process themessage protocol and in which the timeslot access method according tothe invention is used. To this end, the principle of synchronizationbetween a master and a slave will be described first of all as is shownin FIG. 2, in the form of a timing diagram with counter values n plottedagainst time t. Each slave has a timer which is set to a specific value(Nsync) by the reception of a specifically defined slave-specificmessage (for example a synchronization message which may also transportuser data) from the master relating to the synchronization time (Tsync).This value is calculated in advance by the master and is reportedtogether with the overall cycle time (Tcycl) to the slave in aninitialization phase. This method allows any possible brief failure ofthis slave-specific synchronization message to be bridged, since thecounter starts a new cycle once the overall cycle time (Tcycl) haselapsed, even without any need to be corrected in the meantime by meansof the synchronization message.

[0041] This is seen in FIG. 2, where the profile of a clock signal nwith the overall cycle time (Tcycl) is plotted against time t. If thesynchronization signal for the slave now fails for a number of cycles,then the timer continues to run autonomously. In this case,discrepancies can occur between the overall cycle clock (Tcycl) and thetimer-internal clock, for example due to crystal drift in the counter.Such a clock error is shown in the form of a dotted line. Any error Δwhich is present at the synchronization time (Tsync) can be correctedwhen a synchronization signal is once again available, and this resetsthe timer.

[0042] The crystal drift between a master and slave (for example ±100ppm, crystal characteristic) together with the cycle time (Tcycl) of thecyclic data traffic governs the accuracy limits of the timeslots foraccess control. In this case:

[0043] cycle time×crystal drift>maximum accuracy of the timeslot(example: 1 ms×±200 ppm=±0.2 μs>maximum accuracy of the timeslot).

[0044] Each communication network KOMSYS1, KOMSYS2 of networksubscribers consists of one, and only one, master unit and one or moreslave units and, according to the invention, is operated for cyclic datainterchange using a timeslot access method, in order to achieve optimumdead-time data transmission.

[0045] In an initialization phase, in which only the master may transmitand only the slaves may respond, each slave receives the information onthe times at which it will receive messages from the master and on thetimes at which it can send its message or its messages. The precisedefinition of the times at which the messages must be sent or receivedallows the communication to be controlled both at the master end and atthe slave end. FIG. 3 shows the sequence of such a timeslot accessmethod according to the present invention.

[0046] Specifically, FIG. 3 shows the access of a master unit MS to twoslave units SL1 and SL2 within one overall cycle (Cycl) or (Tcycl). Oncea first time Tsend1 has elapsed and after the start of the cycle, themaster MS sends a transmitted message Ssl1 to the slave SL1. Owing tothe delay times on the communication path, this message arrives at theslave unit SL1 as a received message Rsl1 with a certain time delaydtDel(sl1).

[0047] The interval between the end of the received message and thestarting point of the respective overall cycle (Cycl) represents thereception time Trec(sl1) for the slave SL1, which corresponds to thespecific synchronization signal Tsync1 for the slave SL1. At this time,the slave SL1 always receives its synchronization signal and its data.This time represents the clock time for the slave unit SL1, and itstimer is set to a value corresponding to the time Tsync1. The slave SL1is thus assigned the timeslot associated with this for reception ofmessages. The timer is, for example, decremented, with the set valuebeing designed such that the counter zero crossing is coincident withthe end of the overall cycle. The slave SL1 now sends its responsemessage Sms1 to the master, to be precise exactly after a transmissiontime Tsend(sl1) for the slave SL1, measured from the start time of therespective overall cycle. This transmission time Tsend(sl1) is allocatedin a fixed manner to the slave SL1, and represents its timeslot forsending messages. This message once again arrives with the delaydtDEL(Sl1) which is governed by the delay time in the communicationpath, as the received message Rms1 at the master unit MS. The timeinterval from the start time of the respective overall cycle (Cycl) andthe end of the received message Rms1 represents the reception time Trec1for the response from the slave SL1 from the point of view of the masterMS.

[0048] The communication between the master MS and the second slave unitSL2 takes place on the basis of the same fundamental procedure. Once afurther time Tsend2 has elapsed after the cycle start, the master MSsends a further transmitted message Ssl2 to the slave SL2. Owing to thedelay times in the network, this message once again arrives as thereceived message Rsl2 at the slave unit SL2 with a certain time delaydtDel(sl2).

[0049] The interval between the end of the received message and thestart time of the respective overall cycle (Cycl) represents thereception time Trec(sl2) for the slave SL2. This corresponds to thespecific synchronization signal Tsync2 for the slave SL2. The slave SL2always receives its synchronization signal and its data at this time.This time thus represents the clock time for the slave unit SL2 in acorresponding manner, and its timer is set to a value corresponding tothe time Tsync2.

[0050] The slave SL2 is thus assigned the timeslot associated with thisfor receiving messages. The timer is, for example, likewise decremented,with the set value being designed such that the counter zero crossing iscoincident with the end of the overall cycle. The slave SL2 sends itsresponse message Sms2 to the master MS, to be precise exactly after atransmission time Tsend(sl2) for the slave SL2, which is measured fromthe start time of the respective overall cycle. This transmission timeTsend(sl2) is allocated in a fixed manner to the slave SL2 andrepresents the timeslot for sending messages. The slave unit SL2 cantherefore start to send a message Sms2 to the master MS only beforecomplete reception of the message Rsl2, since the corresponding timeslot for sending after Tsend2 is already known from a previous period ofthe overall cycle (Cycl), and the slave unit SL2 is thus already in thesteady synchronization state.

[0051] This message also arrives with the delay dtDEL(sl2), which isgoverned by the delay time on the communication path, as the receivedmessage Rms2 at the master unit MS. The time interval from the starttime of the respective overall cycle (Cycl) and the end of the receivedmessage Rms2 represents the reception time Trec2 for the response fromthe slave SL2 from the point of view of the master MS.

[0052] The procedure for the other slave units likewise correspond witheach slave unit being allocated its own exclusive timeslot. Hence, it ispossible to immediately identify not only interference, but also delayedor premature transmission of a message. A specific message to a slave isused for synchronization in addition to data transmission.

[0053] Apart from timeslot control, the synchronization also means thatthe actual values can be stored in all the slaves at a specific commontime, in this case the start of the cycle (Cycl), in order to achievesimultaneous, equidistant sampling for the controller R. To this end,the respective actual value from each slave unit, in particular from thesensors and transmitter systems S1 and S2, is stored when thecorresponding timer reaches its zero crossing, and is then transmittedto the higher-level master unit NC or R. In the case of the drive systemwith two coupled axes shown in FIG. 1, the slave units S1, S2 supplycorresponding rotation speed, position and orientation actual values.

[0054]FIG. 4 shows one possible message frame for a message T to beinterchanged via the network. The message starts with a message preamblePR, the length of which is, for example, 64 bits. This is followed by afunction code sequence FCS of 16 bits, which is explained in thefollowing text with reference to FIG. 5. This is followed by 8 bits withthe address SLA of the slave unit. In this case, the number of bitsrequired depends on the number of slave units to be addressed. If thereare 8 bits, the number of slaves which can be addressed is 255 (thevalue ZERO is normally reserved for a broadcast function). This isfollowed by the length LE of the subsequent user data DU. The messagelength is thus variable. The message frame ends with a checksum CRC witha length, for example, of 32 bits (CRC stands for cyclic redundancycheck, a known method for error monitoring).

[0055]FIG. 5 shows one possible detailed view of the section comprisingthe function code sequence FCS. The most significant bit contains asupervisor bit SV. This represents a sign of life such that, for exampleif the control software crashes, it is no longer set in the master. Thisallows, for example, a change to a safe state to be initiated in theslave.

[0056] The bits 14 to 10 are reserved for any desired tasks. They arefollowed by three control bits CTRL0 to CTRL2, an area of three to sixbits for storing the message type TY and a further bit LI which signalswhether any length information is present or whether a fixed,standardized message length can be assumed. The bit M/S signals whetherthis is a message which is being transported from a master to a slave orvice versa, and thus indicates the transport direction. The last bitSYNC indicates whether this is a synchronization message or a usermessage. It is also feasible for the SYNC bit to be used to signal thatthe message contains additional synchronization data as well as userdata.

[0057] The control bits CTRL0 to CTRL2 are used, for example, to providespecific security or safety functions, such as those which are required,in particular, in the field of industry automation. One possibleimplementation of a security or safety function with the aid of thecontrol bits CTRL0 to CTRL2 is shown in FIG. 6 which shows a blockdiagram with the internal construction of a slave unit, for example ofthe power section A1, from the drive controller shown in FIG. 1.

[0058] If the module Kom, which receives the message via the line driverPHY and implements the communication protocol, is independent of amicroprocessor μP in the slave application (the actual power section L),specific application events can be initiated in the slave A1 by means ofthe control bits CTRL0 to CTRL2 without requiring the microprocessor μPand the corresponding software in the slave A1. This corresponds to asecond initiation channel K2 as is required for certain security andsafety applications (for example emergency stop etc.).

[0059] The above method steps according to the invention allow acommunication network with a deterministic time response, and hence thecapability for real-time communication, to be achieved, in particularfor industry automation as well, on the basis of Ethernet physics.

We claim:
 1. A method for real-time communication between a plurality ofnetwork subscribers in a communication system using Ethernet physics,comprising having a master unit and at least one slave unit communicatewith one another by means of messages which are transmitted via thecommunication system; interchanging the messages cyclically withequidistant sampling times so that each slave unit is synchronized tothe master unit by means of a common timebase; and using a timeslotaccess method as access control for transmission and receptioncommunications between the network subscribers.
 2. The method accordingto claim 1, further comprising synchronizing the slave units to themaster unit so that each slave unit is clocked via a respective timerwith a predetermined overall cycle time, and wherein the respectivetimer is set cyclically by the reception of a respective slave-specificsynchronization information item which is determined by the master unit.3. The method according to claim 2, wherein user data messages andspecific synchronization messages which contain the respectivesynchronization information items are transmitted.
 4. The methodaccording to claim 2, wherein the synchronization information items areintegrated in appropriately identified user data messages.
 5. The methodaccording to claim 2, wherein the timer in a slave unit automaticallystarts a new cycle once a predetermined overall cycle time has elapsed,even in the absence of synchronization information.
 6. The methodaccording to claims 2 and 5, wherein only the master unit hastransmission authorization in the communication system forinitialization and reports to each slave unit, and each slave unit hasonly response authorization, via an appropriate slave-specific message,wherein the overall cycle time has time slots in which the slave unitwill receive messages from the master unit and timeslots in which theslave unit should send messages.
 7. The method according to claim 6,wherein the slave unit is given a respective synchronization time in aninitialization phase.
 8. The method according to claim 2, whereininstantaneous values are stored in a slave unit at a common time.
 9. Themethod according to claim 1, wherein with each message a slave unitsends a signal to the master unit, and the master unit stops that slaveunit in a controlled manner in the absence of said signal.
 10. Themethod according to claim 1, wherein monitoring information is providedin each message which is transmitted by the master unit to a slave unit,by means of which security or safety functions which are provided in theslave unit can be activated directly via a second initiation channel.11. The method according to claim 1, wherein the master unit sends amaster life signal in each of its messages to each slave unit and, inthe absence of this signal, each slave unit automatically reacts bystopping its own functions in a controlled manner.
 12. The method inclaim 1, wherein separate transmitting and receiving lines between twonetwork subscribers are used simultaneously, so that all the slave unitstransmit only in the direction of the master unit and receive onlymessages from the direction of the master unit.
 13. A method forreal-time communication between network subscribers in a plurality ofcommunication systems using Ethernet physics, wherein, within eachcommunication system network subscriber communicates with one another inaccordance with the method of claim 1, and further wherein a number ofnetwork subscribers have one circuit part in order to form networknodes, said circuit part being used for passing messages in thedirection of another master unit or slave unit, and whereincommunication between network subscribers via network nodes is also inaccordance with the method of claim
 1. 14. A communication system usingEthernet physics for carrying out real-time communication in accordancewith the method of claim
 1. 15. A hierarchical network havingpoint-to-point connections, which are connected via network nodes, usingEthernet physics for carrying out real-time communication in accordancewith the method of claim
 1. 16. A distributed drive system having ahierarchical network according to claim 15, wherein a firstcommunication system comprises a numerical movement controller as themaster unit and at least one control unit as the slave unit, with eachcontrol unit being used as the master unit for a further communicationsystem, which has at least one power section for driving a motor, and anassociated transmission system (S1, S2) as slave units.